by Rachel and Elie Diner
One of the most important questions for any microbe is where their next meal will come from. For us, whether to make your own meal or eat food prepared by others is a common, perhaps daily, dilemma. After a long day of work, take-out might sound pretty good. Or, if you’re a poor graduate student, maybe cooking is a better option. In the natural world, the dine-in vs. take-out dilemma is played out in a different form: namely, do organisms make their own food by means such as photosynthesis, or do they rely on consuming other organisms or pre-made organic compounds for sustenance? Regarding organic carbon, organisms do typically either one or the other. For example, plants and most algae photosynthesize, while animals such as ourselves consume this "pre-made" organic material through eating plants or other animals. This is the autotroph vs. heterotroph distinction. As it turns out, however, many microbes (as well as a select few multicellular organisms) can get the best of both worlds, utilizing a "mix" of different trophic strategies. These organisms are known as mixotrophs.
Consider an organism capable of both acquiring or producing organic carbon, such as a simple sugar. Such an organism can both make organic carbon from an inorganic carbon source or sources (again, such as photosynthesis) and take up organic carbon from the environment. This is not an uncommon situation; many plants and algae have this lifestyle. Furthermore, organisms can sometimes make the organic carbon they need, but have to "eat" other nutrients, such as nitrogen, phosphorus, or certain vitamins. In any case, microbes are extremely resourceful when it comes to how they obtain their food, and some have developed (as we’ll discuss below) some fairly intricate and unique mechanisms for doing this. There is also a distinction between how the food is taken up by the organism. Dissolved carbon or nutrients can be absorbed through the cell membrane (referred to as osmotrophy or saprotrophy). Particles and prey can also be engulfed via the process of phagotrophy, digested extracellularly, or consumed through other feeding strategies.
For mixotrophs, are either or both trophic modes required for survival? Can they mix and match whether they make their own food or obtain it elsewhere? Is one mode required, is another optional? When one trophic mode is required for survival, it is "obligatory," and when it is not required but is still utilized it is "facultative." For example, a plant that must always photosynthesize but has the option to "hunt" for food to gain a fitness advantage is an obligate phototroph and a facultative heterotroph (think of the Venus fly trap). Thus, mixotrophy may be either facultative or obligate for one or more of the trophic modes. Because of the diversity of microbial life, an example of pretty much any of these combinations can be found somewhere on Earth.
In addition to some amazing examples of mixotrophy found among microbes, which is the primary focus of this article, the idea of defining an organism based on its mode of carbon and nutrient acquisition leads to a very interesting grey area. Depending on your vantage point in evolutionary history, you might actually be looking at a symbiotic relationship on its way to becoming a single mixotrophic organism (a mixotroph in the making). Additionally, an organism can be a "functional" mixotroph by means of using other organisms to produce energy for it, such as in the case of plastid-thieves and their so-called "kleptoplasts."
The hunters and (light) gatherers
A particularly exciting example of microbial mixotrophs are the dinoflagellates. This incredibly diverse group of protists can have lifestyles ranging from free-living to parasitic to symbiotic. About half of them are photosynthetic. They take on a variety of rather strange looking forms (Figure 2); they have two flagella in a unique formation (one wrapped around a groove in the middle, and one on the bottom) and many have a thick cellulose cell wall known as theca. They play an important role in many aquatic ecosystems and are "microbially famous" for many reasons, including interesting physical features such as the ocelloid eye (a recent STC subject), forever compact histone-less chromosomes their large complex genetic lineage resulting from multiple endocytotic events (Figure 3), and their common toxicity (red tides that cause shellfish poisoning, and some kinds of respiratory illnesses). Some dinoflagellates, such as those in the Symbiodinium genus, are also symbionts of marine animals such as corals, where they expand the metabolic capacity of their hosts.
Dinoflagellates, like all? Microalgae, evolved from non-photosynthetic protistan ancestors that acquired photosynthetic capacity by engulfing a cyanobacterium, either directly or indirectly. While some microalgae evolved from directly engulfing the cyanobacterium and then retaining their plastids (primary endocytosis), other algae indirectly acquire these plastids by secondary (engulfing one of these "engulfers") or even tertiary endocytosis (engulfing the engulfers’ engulfers…Bear with us, it’s complicated- Figure 3). Dinoflagellates fall into this latter category, and some lineages are the result of ancestral plastids being substituted several different times. ). This tendency to engulf others stuck with the dinoflagellates, and many of them are capable of phagocytosing prey whole, as well as photosynthesizing. Others have since lost the ability to photosynthesize, and rely instead on either ingestion of prey alone or kleptoplasty (stealing chloroplasts). You may have recently heard of the slug that does the same thing. The ability of dinoflagellates to explore and combine trophic strategies has allowed their expansion into diverse ecological niches.
Dinoflagellates prey on a wide variety of organisms including bacteria, heterotrophic protists, algae (including other dinoflagellates), and even small animals. Dozens of photosynthetic dinoflagellate species have been reported to be mixotrophic, and as many of these observations were only recently made, there are likely many more with this skill. They utilize a diverse range of feeding mechanisms. Some generate water currents using their flagella, or a cellular extension, a pseudopod, to draw prey into the cell through a central groove called a sulcus. Others, including several Protoperidinium species, employ a feeding strategy known as pallium feeding, where a modified pseudopod which is called a pallium, or "feeding veil," is extended to engulf prey and digest them extracellularly (Figure 4). This veil can expand up to 10 times the diameter of the dinoflagellate cell. The feeding strategy is used to consume large prey, including entire chains of diatoms (so much for loving your fellow phytoplankton), and the record "catch" observed for this feeding method is over 60 diatoms! Lastly, others utilize a feeding-tube-like structure called a peduncle to attach to prey and basically suck out their guts (think here of the brain bug from the Starship Troopers). Pretty awesome.
For several reasons, it can be difficult to study mixotrophy experimentally. One is that the prey as well as the predator can be quite small, making observation difficult (some dinoflagellates can consume single bacterial cells). Another is that feeding might be induced only by hard to fathom particular conditions. To add to this, many dinoflagellate species have not been cultured, making experimental observation difficult. Despite these problems, some things are known about some common feeding modes. Some of species will only ingest prey when organic nutrients are limiting (e.g. Ceratium furca and Prorocentrum minimum), and addition of nutrients to nutrient poor medium inhibits their feeding. Others will feed even when nutrient conditions are replete. Some Dinophysis species can photosynthesize but cannot survive without some prey ingestion, making them obligate heterotrophs. An excellent test of whether a photosynthetic dinoflagellate is actually a mixotroph (barring visual evidence of it feasting on unfortunate prey) is to see whether it can grow in the dark. The growth rate of the dinoflagellate Fragilidium subglobosum increases at many different light intensities (the most dramatic affect being observed in low light levels) when prey is provided, and can grow quickly in the dark when provided with healthy prey. Although it can grow equally well as a phototroph or heterotroph, it grows the fastest as a mixotroph.
Cyanobacteria are no exception to the mixotrophy game, in fact, the most abundant cyanobacteria in the marine environment, Prochlorococcus marinus is mixotrophic. Recently, it was shown that several laboratory strains of Prochlorococcus are able to take up glucose (Figure 5). This is actually quite rare for many laboratory isolated strains of cyanobacteria for a variety of reasons, including toxicity, lack of metabolic pathways for reduced carbon compounds, or lack of transport systems. Previously, Prochlorococcus had been shown to take up organic nitrogenous compounds, yet had been thought to be photoautotrophic for carbon assimilation. This notion was overruled by the identification of a putitive transporter of glucose that is highly upregulated during glucose exposure. When this transporter, Pro1404, is expressed in cyanobacteria that cannot uptake glucose naturally, these cells are now granted the ability to uptake radiolabeled glucose (Fig 5). This transporter is able to import glucose even at very low nanomolar concentrations, almost identical to those found in the open ocean, suggesting that it evolved in low glucose environments, thus providing Prochlorococcus with a selective advantage. Does this explain why this cyanobacterium is the most abundant marine microbe?
The symbionts and the fakers
No organism exists in isolation, and many unicellular and multicellular life forms to rely on other organisms to expand their trophic options, thus enhance their survival. One common example includes marine animals such as corals, sea anemones, and clams, which recruit and maintain endosymbiotic photosynthetic dinoflagellates of the genus Symbiodinium (also known as zooxanthellae). The carbon fixed by the dinoflagellates supplements the food that these animals are able to eat via filter feeding. While this is an interesting example of a trophic expansion, is it mixotrophy? Mixotrophy typically refers to the "eating habits" of a single organism, however, as we increasingly realize just how interconnected different life forms are, this distinction becomes blurry. These relationships also change over evolutionary time, and at one point mixotrophs may have been a consortium of organisms playing different roles together. At some point, the endosymbiont may become an organelle, and the ectosymbiont an autotroph or a mixotroph. For this reason dinoflagellates make a particularly interesting model for studying organelle acquisition.
Another group of microbes, mainly protists, appear to have acquired the ability to make their own food but may (or may not) be "faking" being photosynthesizers by using stolen plastids. A prior STC article examined this phenomenon in sea slugs that eat algae and then keep their plastids around to temporarily photosynthesize. Though these plastids last for a little while, they eventually need to be replenished. This can occur among microbes as well. Some ciliates for example can achieve the same feat, benefitting from their prey’s photosynthetic capacity. But what happens when it’s difficult to tell whether the chloroplasts belong to the predator or the prey?
This may be the case for the Dinophysis dinoflagellates mentioned above. This species must consume prey to survive, which in all studies has been the ciliate Mesodinium rubrum, and is considered an obligate mixotroph. The question, though, is whether it has its own chloroplasts or is actually using those of its prey. Molecular evidence suggests that the chloroplasts of the dinoflagellate and of M. rubrum are identical in DNA sequence. On the other hand, microscopic evidence suggests that the structure of chloroplast components of predator and prey is actually quite different, leading some to refute the kleptoplasty designation. While this topic is still debated, it demonstrates that it is sometimes not quite clear what an organism’s trophic strategy really is. In the case of dinoflagellates, many species seem to exist on some sort of continuum between being permanently photosynthetic, "borrowing" photosynthetic capabilities, and dispensing with photosynthesis altogether.
With the seemingly endless diversity of microbes, it is not surprising that their ways of obtaining nourishment are equally varied, and that employing multiple types of feeding strategies can provide a competitive advantage in the fight for survival. Of course, there are tradeoffs to maintaining a particular strategy. For example, photosynthesis is expensive to maintain and some organisms have adapted to using a single feeding mode only, while others either can or must rely on multiple strategies, depending on their niche. Dinoflagellates are an excellent example of this diversity, which has led to important associations with other organisms as well as great ecological success as free-living organisms. Thus, while we are left at the end of the day deciding between a burger at In-N-Out or a salad at home, the dinoflagellates have already decided: they’ll have both!
Rachel is a Ph.D. student in the Allen lab at the Scripps Institution of Oceanography and the J. Craig Venter Institute in La Jolla, CA. She studies marine microbes and how they interact with each other, humans, and the environment. In her free time she enjoys surfing, cooking, and reading.
Elie is a postdoctoral fellow in the Romesberg lab at The Scripps Research Institute in La Jolla, CA. He is interested in synthetic biology, expansion of the genetic code and microbiology. In addition to science, he enjoys surfing, camping and spending time outside.